Precambrian intraplate magmatism: high temperature, low pressure crustal granulites

Starting from the hypothesis that the lower continental crust is dominated by granulites, some essential features of granulites are recalled. Two gene ral P-T-t paths are possible, clockwise and anticlockwise, of which the for mer is more common for high pressure granulites, the latter for low pressur e granulites. For high temperature, low pressure granulites, granulite meta morphism appears to be a discrete event, caused by a sudden temperature inc rease in a more or less thickened crust. The complex granulite chemistry, c haracterised by the possible mobility of some elements (LILE, Ti), as well as the P-T-t evolution, are explained by two complementary mechanisms opera ting simultaneously in different domains and at a variable scale: fluid-abs ent melting and fluid-assisted metamorphism (high salinity aqueous fluid an d high density CO2 fluid). CO2 is brought into the lower crust by mantle-de rived magmas, which also supply the required heat, whereas the high salinit y aqueous fluid is probably derived from internal crustal sources. The size of Precambrian granulite provinces, the geodynamic setting, and the chemic al composition of syn-metamorphic intrusions, do not suggest a direct relat ion with subduction processes. This paper favours a model of intraplate ext ension without rupture, ultimately leading to a vertical accretion of the c ontinent by the incorporation of mantle-derived magmas at the base of the c rust. (C) 1999 Elsevier Science Limited. All rights reserved.